Radio Frequency Identification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave may be far field, in which the electric- and magnetic-field components of the underlying electromagnetic field have an amplitude ratio that depends on the propagating medium. The RF wave may alternatively be near field, in which one of the electric or magnetic field components can be dominant.
A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna system, a radio section, a power management section, and frequently a logical section, a memory, or both. In some RFID tags the power management section includes an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or battery-assisted tags. Advances in semiconductor technology have miniaturized the electronics sufficiently that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device such as a battery, and are called passive tags. Regardless of the type, all tags typically store or buffer some energy temporarily in passive storage devices such as capacitors.
Many parameters of RFID communications such as the amount of energy that a tag can extract from an electromagnetic field, the tag's communication data rate with a reader, the communications protocol, the maximum range of the communications, the tag antenna performance relative to its orientation in the electromagnetic field, and the scattering and interference environment, vary with frequency. In some instances it may be desirable for an RFID tag to operate at multiple frequency ranges sequentially or simultaneously. In other instances it may be desirable for an RFID tag to operate at one frequency range while extracting energy from an electromagnetic field at a different frequency range. In yet other instances it may be desirable for an RFID tag to operate at one frequency range while monitoring another frequency range.